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It is 21 years since the nuclear plant at Chernobyl went bang, and the extent of the damage wrought by the radioactive fallout is still becoming clear.

According to a report in Chemistry World, US researchers have discovered that Swedish children who were in the womb at the time of the accident might have been mentally damaged by their exposure.

The findings, which suggest the developing foetus may be more sensitive to radioactivity than previously thought, are based on a survey of the academic achievements of more than 560,000 Swedish children born between 1983 and 1988.

The researchers, economists Douglas Almond and Lena Edlund from Columbia University, New York, US, and their Stockholm University colleague Mårten Palme, found that foetuses of between eight and 25 weeks post-conception at the time of the accident were most affected, academically.

Locations counts too, with children born in the regions worst affected by the radioactive fallout faring worst of all.

Interesting as the data are, there are plenty of questions still to be answered. The levels of exposure were lower than those thought to be "safe", based on studies of Japanese children affected by the Hiroshima and Nagasaki bombs in 1945. So how such a relatively low dose could cause significant impairment remains to be explained.

However, the researchers are clear that they have done no more than demonstrated a statistical link: they have not proven that the fallout from the explosion has directly caused mental impairment.

David Brenner, professor of radiation oncology at Columbia University described the work as "suggestive", while Jim Smith, an environmental scientist specialising in the effects of the Chernobyl disaster at the University of Portsmouth, told Chemistry World that the study seemed to have done a good job at controlling "unknown factors" that often confound such research.

The nuclear power plant at Chernobyl exploded on 26th April 1986. Safety systems were switched off to allow the crew to run a series of tests, but things didn't go quite according to plan.

The reactor's power was supposed to be switched to a quarter of its normal output for the test but instead fell to just one percent. When the engineers tried to ramp up the power, there was an unexpected surge. Then the emergency shutdown failed, and the reactor was out of control.

Most of the radiation from the explosion was released in the first ten days, when the prevailing winds were northerly and north-westerly. By the end of April, the winds had shifted to the south, meaning the fallout was spread far and wide. The reactor was finally capped with a concrete "sarcophagus" in November 1986. ®